Systems for measurement of surfaces of objects are e.g. used in industry in production processes for checking component and product geometries. This enables design tolerances of objects to be checked and production-related manufacturing defects to be rapidly identified and parts whose dimensions lie outside a predefined standard to be removed from the process. An improvement of a production quota with a simultaneous reduction of the production errors of parts can result from the use of a measuring system of this type.
In an industrial manufacturing process e.g. measuring systems with tactile sensors can be used. Said systems mainly comprise a movable, guided measuring tip with a ruby ball attached to its end, wherein when there is contact of the ruby ball with the object surface a measurement point is detected and the coordinates of the point can be determined. By scanning defined object positions or by retraction from parts of the surface the shape and dimensions of the object can be recorded, compared with target values determined in advance for this purpose and any deviations from the target values can be determined.
The tactile measurement of objects can be disadvantageous for very large objects, e.g. aircraft, because such a measuring process is very time intensive on the one hand and because of the size of the object can only be carried out with difficulty on the other hand. The increasing requirements for substantially complete quality control during a running production process and for the digitization of the shape of objects, in particular of prototypes, make the recording of surface topographies—in relation to determining coordinates of individual points of the surface of the objects to be measured in a short time—an ever more frequently posed measuring task.
To reduce the measuring time for this purpose an optical measurement sensor can be used for contactless measurement. Such an optical surface measuring system can in general comprise a measuring system employing image sequences to determine 3D coordinates or an optical scanner, e.g. a line scanner, with which the surface is scanned line by line, wherein the measuring system or the scanner is simultaneously guided over the surface.
A scanner that is known from the prior art can record the distances to a plurality of points in a short time depending on the respective scanning angle. By taking into account the movement of the scanner relative to the surface a scatter plot can be generated therefrom, which represents the surface of the object to be measured. By using image processing a measured surface can also be graphically processed, represented on a display and an indication of any measurement deviations occurring can be merged into the graphic.
Furthermore, measuring systems using image sequences to determine 3D coordinates of measurement objects that are known from the prior art, which e.g. can be in the form of portable, hand-held and/or permanently installed systems, comprise in general a pattern projector for illuminating the measurement object with a pattern and are thus sometimes referred to as pattern-projecting 3D scanners or light structure 3D scanners. The pattern projected onto the surface of the measurement object is recorded by a camera system as a further component of the measuring system.
During a measurement the projector illuminates the measurement object time sequentially with different patterns (e.g. parallel light and dark strips of different widths, in particular a rotation of the strip pattern can also occur, e.g. by 90°). The camera(s) record(s) the projected strip pattern at a known perspective to the projection. An image is recorded for each projection pattern with each camera. For each image point of all cameras there is thus a time sequence of different brightness values.
Other suitable patterns can also be projected besides strips, however, such as e.g. random patterns, pseudocodes, etc. Suitable patterns for this purpose are sufficiently known to the person skilled in the art from the prior art.
Pseudocodes enable e.g. an easier absolute association of object points, which is increasingly difficult for the projection of very fine strips. For this purpose initially one or more pseudocodes and then a fine strip pattern can be projected in a rapid sequence or even in successive recordings different strip patterns that become finer during the sequence can be projected, until the desired accuracy is achieved in the resolution of measurement points on the measurement object surface.
The 3D coordinates of the measurement object surface can then be calculated from the recorded image sequence using image processing according to a method known to those skilled in the art in this area of photogrammetry and/or strip projection. For example, such measurement methods and measuring systems are disclosed in WO 2008/046663, DE 101 27 304 A1, DE 196 33 686 A1 or DE 10 2008 036 710 A1.
A problem with carrying out a measurement with a measuring system with a camera or scanner often arises with manual, i.e. hand-held, measurements. Because in principle very high computing powers are necessary in order to record and to process a scatter plot of a surface in three-dimensions, in particular if recorded images or scan lines are to be merged because of a movement of the measuring system, the movement tolerance is usually significantly limited in relation to speed and vibration. A substantial steadiness of the measuring system when measuring is thereby mandatory and if not maintained is a major cause of generated measurement errors.
One approach to taking such measurement errors into account when detecting the surface—even when using a robot arm to guide the measurement sensor—and thus to compensate for vibrations or speed changes during the measurement is e.g. disclosed in the European patent application with the application number 10166672.5.
When recording image sequences, e.g. translational and/or rotational accelerations of the measurement sensor or the measurement object can be measured and the measured accelerations can be taken into account when determining the 3D coordinates of the object. Determining the 3D coordinates of measurement points can take place depending on the measured accelerations. Thus errors caused by unsteady guidance of the measuring system can be taken into account in a computer and as a result correct position data can be determined.
An alternative proposal for making a contactless 3D surface measurement using a recording unit able to be carried out accurately and rapidly is presented in EP 2 023 077. A measuring device coupled to a measuring head can determine a measurement position and orientation of the measuring head for a measurement carried out with the head, wherein the measuring head can be guided to a surface or guided along the same using an articulated arm. By a second measurement of an at least partly overlapping surface section, a surface image can be generated in a common coordinate system from the position information that can be obtained in this way.
A common disadvantage with the above-mentioned embodiments is that with a large object to be measured, because of the limited range of an articulated arm and the anatomically restricted range and mobility of a person guiding the measuring system, the achievement of all relevant measurement positions at the object can only be carried out with difficulty or can be partly not achieved. Furthermore, an object measurement can e.g. only be carried out in a toxic environment by a person in special protective equipment and thus in turn under difficult conditions on the one hand or, on the other hand, the measurement can no longer be carried out e.g. if tolerable toxicity values are exceeded.